You’d think NASA’s Mars Reconnaissance Orbiter (MRO) has seen everything there is to see on the Martian surface in the 11 years it’s orbited our nearest neighbour, but a snapshot taken over the planet’s South Pole has revealed something we can’t explain.

NASA’s Mars Reconnaissance Orbiter (MRO)

Mars. A depression huge and deep on Mars has left astronomers baffled.

The vast pit, estimated to be hundreds of feet across and surrounded by frozen carbon dioxide, is located on the south pole of Mars — sticking out among the Swiss-cheese terrain of Earth’s closest neighbour.

While the planet’s entire surface is pocked with various depressions and craters, a vast pit spotted among the “Swiss cheese terrain” of melting frozen carbon dioxide appears to be a bit deeper than your average hole, leaving astronomers to try and figure out what made it.

The space agency released this photo of a deep “circular formation,” estimated to be hundreds of feet across, which was spotted somewhere near the south pole of the planet. The gigantic pit is surrounded by a sheet of frozen carbon dioxide, which is dotted by more shallow circles that were left in the ice after sunshine caused parts of it to sublimate into gas. This type of land formation is dubbed “Swiss cheese terrain” after its uncanny resemblance to the porous dairy treat.

The researchers have no answer for how the hole got there. But scientists have two top guesses. One is that the hole is an impact crater. NASA has found many of these on the planet. Scientists believe the craters can be used to track the age of different parts of Mars’ surface.

Other explanation could be that a bit of Mars’ surface collapsed. The newly spotted hole is in an area of the planet with “Swiss cheese terrain” –– a nod to the many shallow pits within the carbon dioxide ice. The idea is that one of these pits may have sunk further. No matter what caused it, the hole is likely pretty massive. Some estimate it’s hundreds of meters across.